A common practice in petroleum refining is to blend products from several different process units, such as straight run gasoline, alkylate, hydrocrackate, reformate, oxygenates, etc., and purchased components to obtain gasoline in a grade that meets regulatory requirements such as minimum octane number, Reid vapor pressure, benzene content, and the like. Octane number, as defined by ASTM D2699 for research octane number or ASTM D2700 for motor octane number, is an indication of a gasoline's resistance to pre-ignition during the compression stroke of a piston, and Reid vapor pressure (RVP), as defined by ASTM D323, is a measure of the ease of evaporation of a gasoline and is often indicative of cold start capability.
The blending of various components to form gasoline is a complex process. Depending upon refinery configuration, anywhere from five to twelve or more different base stocks are blended to meet regulatory requirements. Furthermore, the optimization can involve numerous variables including regulated parameters, physical properties and composition of the final blend, and availability and cost of blend components.
Often in industry, the ratios of the components to be blended are determined by charts or mathematical algorithms known as "blending equations". Such blending equations are well known in the petroleum refining art and are developed or adapted by each refiner depending upon available crudes, refinery configuration, and cost and availability of blend stock components. Such equations are most frequently used in the blending of components to form a gasoline which meets regulatory requirements. Examples of blending equations and their applications are given in Cary, J. H.; Handwerk, G. F. Petroleum Refinery Techniques and Economics; Marcel Dekker: New York, 1984; Chapter 11.
Blending equations typically relate a quantity of gasoline, in moles/L for example, at a target value for some characteristic, such as RVP, octane, or percentage composition of oxygenates, to the quantity of each of the component streams multiplied by the measured value of that same characteristic for each component stream. Blending equations also may indicate significant nonlinearity of gasoline parameters with respect to the addition of a blend component. For example, RVP generally varies nonlinearly with the addition of butane or methanol.
Using blending equations is further complicated since the value of a characteristic in a component is not necessarily indicative of the final value of the characteristic in different blended products. For example, the value of a characteristic in the blend may be less than, equal to, or greater than the sum of the proportionate values of the characteristic in the blend components. Stated another way, a component having a high octane, may increase the octane of the final blended product by variable amounts depending on the structure of the additional components being blended. "Blending octane number" is the term typically used to express a refiner's experience as to the impact a petroleum base stock will have on the octane of the final gasoline blend. Unfortunately, the blending octane number may vary, especially when the other components of the blend are changed. Therefore, blending octane numbers are normally estimated by charts, calculated empirically, or measured after the gasoline blending is complete.
Due to the described complexities, blending component ratios are usually estimated based upon laboratory data or experience. Neither approach is entirely satisfactory, and costly overcompensation and even reblending is common in the petroleum industry. The present invention meets the demand for an improved efficient process of controlling the value of a characteristic in a blended composition by using spectroscopic measurements which can be employed on- or in-line and in real-time. For example, even though on-line octane monitors have long been available to monitor a product's octane number, the technology of these monitors is such that each measurement requires at least 12 minutes, and during those 12 minutes the octane number of the product may have changed. Furthermore, controlling blending with these monitors is inefficient due to the 12-minute delay before the effect of each parameter change becomes known. Recently, near infrared (NIR) spectroscopy has been used to determine octane numbers of hydrocarbons thereby providing virtually instantaneous octane numbers since the analysis time is generally only about one minute. U.S. Pat. No. 4,963,745, U.S. Pat. No. 5,223,714, E.P. (0 285 251), E.P. (0 305 090), and W.O. (WO 91/15762) are typical of disclosures which teach that NIR spectroscopy may be used to determine octane number. Many methods of using NIR spectroscopy can be found in the art, and which specific method is applied in the practice of this invention is not as important as the overall contribution of the advantages of NIR spectroscopy. The same pattern of significantly improved response time of NIR spectroscopy as compared to traditional on-line analyses holds for other characteristics as well as octane. Through applying the advantages provided by on-line and in-line NIR spectroscopy in a new efficient process of controlling blending, this invention furnishes a significant cost-reducing alternative to current blending practices in the petroleum industry.
U.S. Pat. No. 4,963,745, E.P. (0 305 090), and W.O. (WO 91/15762) each focused on a specific method for using NIR spectroscopy to measure the value of a characteristic. Each also briefly disclosed that its particular method may be used to control a blending operation. However, in contrast to our invention, none of the disclosures provide the specific details of the control process necessary for one skilled in the art to implement the particular method as a control process. U.S. Pat. No. 5,223,714, disclosed using NIR to predict physical or chemical properties of the blended product from the absorbance of each preblending component. This patent disclosed that such predictions may be used to control blending, but the focus of the patent was on how to make the measurements and how to mathematically manipulate those measurements. Our invention allows any suitable spectroscopic and mathematical techniques to be used and instead focuses on the detailed steps of controlling the blending. E.P. (0 285 251) disclosed an NIR spectroscopic method to determine octane number but did not disclose using the method to control a blending operation.